Precipitation of alumina trihydrate from bayer pregnant liquors

ABSTRACT

In the operation wherein fine seed alumina trihydrate is added as a precipitating agent to Bayer pregnant liquor for the precipitation of alumina trihydrate therefrom there is precipitated a coarser alumina trihydrate, as evidenced by the percentage of precipitated alumina trihydrate having a particle size greater than 325 mesh, by carrying out the precipitation of the alumina trihydrate from said Bayer pregnant liquor in the presence of added calcium carbonate. The increase in the amount of the resulting precipitated alumina trihydrate having a particle size greater than 325 mesh when precipitated in the presence of calcium carbonate is especially evident when the Bayer pregnant liquor has a caustic content measured as Na2CO3 in the range from about 180 grams per liter to about 200 grams per liter and the calcium carbonate is present in the pregnant liquor in an amount in the range from about 0.05 to about 0.10 grams per liter based on the Bayer pregnant liquor.

[451 Sept. 16, 1975 PREcIPITATIoN OF ALUMINA TRII-IYDRATE FROM BAYERPREGNANT LIQUORS [75] Inventor:

[73] Assignee: Alcan Research and Development Limited, Montreal, Canada[22] Filed: Sept. 21, 1971 [21] Appl. No.: 182,481

Bohdan Gnyra, Arvida, Canada [52] US. Cl. 423/629; 423/625; 423/630;

423/127; 423/121 [51] Int. Cl. COlf 7/02 [58] Field of Search 423/625,629, 630, 127

United Kingdom Reese et al., Industrial and Engineering Chemistry,

Vol. 47, No. 9, pp. 16724680, 1955 Linke, Solubilities of Inorganic andMetal Organic Compounds, 4th Ed., Vol. I, p. 547, (1958).

Primary ExuminerHerbert T. Carter Attorney, Agent, or FirmCooper,Dunham, Clark, Griffin & Moran [57] ABSTRACT In the operation whereinfine seed alumina trihydrate is added as a precipitating agent to Bayerpregnant liquor for the precipitation of alumina trihydrate therefromthere is precipitated a coarser alumina trihydrate, as evidenced by thepercentage of precipitated alumina trihydrate having a particle sizegreater than 325 mesh, by. carrying out the precipitation of the aluminatrihydrate from said Bayer pregnant liquor in the presence of addedcalcium carbonate. The increase in the amount of the resultingprecipitated alumina trihydrate having a particle size greater than 325mesh when precipitated in the presence of calcium carbonate isespecially evident when the Bayer pregnant liquor has a caustic contentmeasured as Na CO in the range from about 180 grams per liter to about200 grams per liter and the calcium carbonate is present in the pregnantliquor in an amount in the range from about 0.05 to about 0.10 grams perliter based on the Bayer pregnant liquor.

7 Claims, 1 Drawing Figure fFFEcr 0/-' 6 450 0,11 PART/c4: 5/25 a;ALuM/A/A Lemma/e475 pREC/P/TAT/M/ fieaoucr 114/9652 7mm zoo Mes/r) 8:

Peoaocr $725 [9; F6

l I I l I 0a ('0 A0050 m HAL/A16 new? (gm: P5? 0725) drate isprecipitated from the resulting filtered Bayer pregnant liquorby theaddition thereto of finely divided seed alumina trihydrate as .aprecipitating agent for the precipitation of the alumina trihydrate-fromthe Bayer pregnant liquor. I V

It is desirable to produce an alumina trihydrate hav= ing a coarse orrelatively large particle size so as to reduce dust losses duringcalcination and in the potrooms wherein elemental aluminum is producedby electrolysis of alumina. For example, it is especially desirable whenalumina trihydrate is precipitated from Bayer pregnant liquor to produceor to precipitate an alumina trihydrate produce having a particle sizesuch that no more than about"6% by weight of the precipitated aluminatrihydrate has 'a particle size smaller than 325 mesh.

Accordingly, it is an object of this invention to provide an improvedprocess for the precipitation of alumina trihydrate from Bayer pregnantliquor produced by the digestion of alumina-contaningore, e.g. bauxite,with an aqueous caustic (NaOI-I) solution.

It is another object of this invention to provide a process for theprecipitation of alumina trihydrate having a relatively large or coarserparticle size upon precipitation from Bayer pregnant liquor in thepresence of added finely divided seed alumina trihydrate.

Still another object of this invention is to provide a process for theprecipitation of a coarser alumina trihydrate product, as evidenced by asmaller percentage of said product having a particle size smaller than325 mesh, when precipitated from Bayer pregnant liquor wherein the Bayerpregnant liquor hasa relatively high caustic concentration, such asabove about 160 grams per liter'(gpl) caustic measured as Na CO Howthese and other objects of this invention are achieved will becomeapparent in the light of the accompanying disclosure made withvreference to the accompanying drawing wherein there is graphicallyillustrated the effect of CaCO on the particle size of aluminatrihydrate when precipitated from a Bayer pregnant liquor. In at leastone embodiment of the practice of this invention at least one of theforegoing objects will be achieved.

It has been discovered that when alumina trihydrate is precipitated fromBayer pregnant liquor in the presence of added fine seed aluminatrihydrate as a precipitating agent for the precipitation of the aluminatrihydrate there is precipitated a coarser aluminatrihydrate productfrom said Bayer pregnant liquor when the precipitation of the aluminatrihydrate is carried out in the presence of an added small amount offinely divided calcium carbonate, such as an amount in the range fromabout 0.02 gram per liter to about 0.20 gram per liter based on theBayer pregnant liquor.

The practices of this invention are particularly applicable torelatively high caustic Bayer pregnant liquors, such as liquors having acaustic content above about I60 grams per liter caustic measured as Na-CO such as Bayer pregnant liquors having a caustic content in the rangefrom about 175-200 gpl caustic and a causticity in the range of about75-90%, more or less. A coarser alumina trihydrate product is obtainablein accordance withithe practices of this invention by adding smallamounts of finely divided calcium carbonate (CaCO to the seeded Bayerpregnant liquors, such as an amount in therange of about 0.05O.IO gramper liter calcium carbonate based on the Bayer pregnant liquorundergoing treatment. The calcium carbonate may be added priorto theaddition of the finely divided alumina trihydrate seed, simultaneouslytherewith, or after the-addition of the alumina trihydrate seed. It hasbeen observed that when the precipitation of the alumina trihydrate iscarried out in the presence of calcium carbonate there is substantiallylittle, if any, reduction in liquor productivity, i.e. in the practicalamount of alumina trihydrate recoverable by' precipitation from theBayer pregnant liquor undergoing treatment.

In the practice of this invention the amount of added CaCO is based onthe Bayer pregnant liquor, i.e. the liquor remaining after filtration ofthe Bayer digest for removal of solid residues, such as the red mud".Thus only filtered clear Bayer pregnant liquor is used for aluminaprecipitation in the practices of this invention.

In the examples set forth hereinafter certainterms well understood bythose skilled in the art are employed. Specifically, total titratablesoda (TTS) isthe sum of the NaOH (measured as Na CO and the true Na COpresent in the liquor. By way of example, a Bayer liquor which contains180 gpl caustic (measured as Na CO along with 60 gpl true Na CO wouldmeasure 240 gpl total titratablesoda (TTS).

Otherfterms, such as causticity and filling liquor ratio, are defined asfollows: I

I. Percent Liquor Causticity" (Liquor Causticity) gpl Caustic Soda gpl x100 gpl Alumina in Solution The End or Spent Liquor ratio is defined inthe same manner, i.e.-

gpl Ammina in Solution gpl Caustic Soda in Solution The followingexamples are illustrative of the practices of this invention and theadvantages obtainable therefrom.

EXAMPLE I TABLE I ALUMINA TRIHYDRATE PRECIPITATION WITHOUT AND WITHADDED CaCO;

End Ratio Product Particle Size (/1) Sample Description (calculated) +74+53 -44 33 Test No. I

TABLE I-continued End Ratio Product Particle Size 2) Sample Description(calculated) +74 +53 V 44 33 Conditions: Plant Filling Liquor: I18.6/189.5 0.626,

T.T.S. 238.5 gpl Seed: Plant fine seed (80% 325 mesh) at 60 gpl chargePrecieitationz24 hrs. at 155F., in duplicate I No CaCO added 0.373 16.271.3 8.3 2.3 2 0.373 23.0 77.1 6.9 2.5 3 0.06 gpl CaCO added 0.373 47.285.8 4.7 1.3 4 0.373 39.5 86.4 3.7 1.0 Test No. 2 Conditions: PlantFilling Liquor: 123.7/190.8 0.648

T.T.S. 240 gpl Seed: Plant fine seed, at 60 gpl charge Precipitation: 24hrs. at 155F., in duplicate 1 No CaCO added 0.363 16.0 72.0 8.0 2.1 20.363 15.0 71.8 8.5 2.2 3 0.06 gpl CaCO added 0.368 V 22.0 76.1 7.0 1.54 0.368 25.2 75.0 7.8 1.4 I

EXAMPLE 2 Additional tests were carried out to demonstrate theadvantages of the practices of this invention wherein calcium carbonatewas added to synthetic Bayer pregnant liquors having varying causticcontent. The results of these tests are set forth in accompanying TableII:

TABLE 1i EFFECT OF ADDED CaCO ON PARTICLE SIZE OF HYDRATE PRODUCTPRECIPITATED AT 150F. for 24 hrs in SYNTHETIC ALUMINATE LIQUORS OF 160,190, and 220 gpl CAUSTIC (as Na,co,,).

230 gpl T.T. Soda, 0.645 Ratios, and Seeded with 66.6 gpl of Bayer FineSeed (80% 325 mesh) Product Particle Size 71 Caustic CaCO, Hydrate +200325 325 Mesh level Added Product Mesh Mesh after Sample (gpl) (gpl)(gpl) (as is) (as is) Attrition N.B.: All CaCO treated samples gavecoarser products.

4 trihydrate product having a particle size smaller than 325 mesh. 1

Additional tests were carried out involving a series of aluminatrihydrate precipitations in Bayer process liquors and in syntheticBayer process liquors made up from caustic pellets, soda ash and aluminatrihydrate digested at 300F, for 30 minutes. These tests and the resultsof these tests are set forth in accompanying Tables III, IV, V and VI:

The particle size of the resulting precipitated alumina trihydrateproducts, after a mild standard attrition (5 minutes recirculation in aminiature centrifugal pump), were compared. It was observed that theCaCO treated samples contained about 4-5 percentage points less of325mesh fines than corresponding samples of alumina trihydrateprecipitated in the absence of CaCO TABLE III EFFECT OF ADDED CaCO ONPARTICLE SIZE OF HYDRATE PRODUCTS PRECIPITATED AT 150F. for 24 HRS INBAYER LIQUORS* OF 812.3 GPL CAUSTIC, 234.5 GPL T.T. SODA AND STARTINGRATIO OF 0.607, SEEDED WITH 60 GPL FINE BAYER SEED ADDED AS 3071 SPENTLIQUOR SLURRY (AT 25 150F.)

Particle Size (/z) CaCO Total +200 Mesh 325 Mesh I added Hydratc (before(after Sample gpl) (gpl) attrition) attrition) *The original soluble CaOin the starting liquid (before seeding) was 0.007 gpl. To this wereadded extra amounts of 0.06. 0.08 and 0.10 CaCQ- TABLE IV EFFECI' OFADDED CaCO ON PARTICLE SIZE OF 40 HYDRATE PRODUCT S PRECIPITATED AT150F. FOR

24 HRS IN BAYER PLANT LIQUORS OF 182 GPL CAUSTIC, 242 GPL T.T. SODA AND0640 RATIO, WITH 78 GPL FINE BAYER SEED (80% -325 MESH) Product SizeCaCO +200 Mesh 325 Mesh added Total Hydrate (before (after Sample (gpl)Product (gpl) attrition) attrition) TABLE V EFFECT OF ADDED CaCO ONPARTICLE SIZE OF HYDRATE PRODUCTS PRECIPITATED AT F. for 24 HRS INSYNTHETIC ALUMINATE LIQUORS OF 160, 190, AND 220 GPL CAUSTIC (AS Na Co230 GPL T.T. SODA, 0.645 RATIO, and SEEDED WITH 66.6 GPL OF BAYER FINESEED (8071 -325 MESH) Product Particle Size (7:)

. 5ft 6 TABLE more uniform,; rounded-off outlines and possessed a morecompact structure. The large number of small EFFECT OF ADDED Caco ONPARTICLE SIZE OF 'monocrystals growingbetween and around large mono-HYDRATE PRODUCTS PRECIPITATED AT l50F for 24 HRS IN crystals seem toexert a strengthening effect on the SYNTHETIC ALUMINATE LIQUORS f 190 a220 whole particle. It would appear that the reduction of GPL CAUSTIC(AS Nazcofi 230 GPL SODA fines in 'CaCO -treated alumina trih drate reciitat RATIO, AND SEEDED WITH 66.6 GPL 6r BAYER FINE a y P P e5 SEEDresults from the better agglomeration of growing seed (80 7e -325 MESH)V particles, the particles being more compact and more Product Size *4)resistant to attrition. Caustic 233 2 3 Product gi t fi f 10 Based ontests carried out, 1t would appear that the Sample (gpl) (gpl) (gpl)attrition) n beneficial effects of CaCO addition to Bayer pregnantliquors for the precipitation of a coarsened alumina tri- 2 160 006133,7 12,6 3L6 hydrate are best observed or achieved,

3 I90 1393 Q I. when the alumina trihydrate seed charge is finely 33 f":32; 533 if divided, SLICI'I as about 80-90% having a particle size 6220 0.06 133.7 l6.0 21.6 smaller than 325 mesh,

2. when high seed charges of about 50-60 gpl and A higher aluminatrihydrate are employed,

In the results of the tests reported in the foregoing ta- 3. when theprecipitation operation from the Bayer bles it is indicated that theeffect of added CaCO is pregnant liquor is carried outwith liquorshaving a low modified by several factors, such as the amount of startingratio, about 0.620,

CaCO added, liquor ratio, and the caustic level of the 4. when thefilling temperatures are relatively low, liquor undergoing treatment. Inthe reported tests the about 150F. and +200 mesh fraction beforeattrition indicates the ex- 5. wherein the precipitation operation iscarried out tent of seed agglomeration whereas the -325 mesh atarelatively high rate of mixing-recirculation. fraction after attritionindicates both the agglomera- Under-the above o eration conditions theaddition of tion and the resistance to attrition of the respective CaCOtends to give 10-15 percentage points less 325 products. mesh fines thanan alumina trihydrate product precipi- The data presented in Table "Ishow a very large partated in the absence of CaCO ticle size differencebetween the alumina trihydrate 3O Satisfactory, but less than optimal,results are products with and without CaCO treatment. The treatachievedwhen ment with CaCO reduced the 325 mesh fraction by l. the aluminatrihydrate seed is relatively coarse, more than 15 percentage points. Inthis test there was about 50%, having a particle size smaller than 325employed wet seeding, i.e. the seed was added as mesh, spent liquorslurry at 150F. Up to 0.08 gpl CaCO 2. lower seed charges, about 30-40gpl, alumina triadded there was virtually no loss in productivity. hdrate are em loyed,

The results in accompanying Table IV referred to a 3, higher startingratios, about 0.65, are employed dry seeding at high charges of 78 gplfine seed alumina with trihydrate to a Bayer pregnant liquor of a highstarting 4. higher filling temperatures, about 160 or higheraluminacaustic ratio of 0.640. In this case the CaCO 40 and treatmentdecreased the 325 mesh fraction in the 5. with gentlemixing-recirculation. Under these less product by about 5 percentagepoints as compared with than optimum conditions the alumina trihydrateprodthe untreated sample. uctwould tend to give only about 5 percentagepoints The data reported in accompanying Tabl s V and I less 325 meshproduct as compared with correspondindicate that in Bayer pregnantliquors having between ing alumina trihydrate precipitated in theabsence of I and 190 gpl caustic the addition of 0.06 gpl CaCO calciumcarbonate. reduced the -325 mesh fraction in the produ t y Furtherillustrative of the advantages of the particles about 5 percentagepoints at the expense of a relatively of thi invention and su ortive ofthe above, addismall decrease in liquor productivity. tional tests werecarried out. These tests and the results Upon examination of the aluminatrihydrate precipitated in the presence of calcium carbonate there wasobserved that the alumina trihydrate particles exhibited of these testsare set forth in accompanying Tables VII, VIII and IX.

TABLE vn COMPARATIVE PRECIPITATIONS WITH AND WITHOUT CaCO PrecipitationConditions:

(i) Wet seeding: Bayer Fine -Seed 325 mesh added as 307:

slurry in spent liquor (ii) 0.06 gpl CaCO added as indicated.

(iii) Precipitated at F. for 24 Hrs.

(iv) Filling Liquor: I l9. 7/l86.3 0.643; T.T.S. 2.359 gpl.

(v) Spent Liquor (with seed): 640/1880 =0.340; T.T.S. 242.5 gpl.

cc Filling liquor productivity 7 in 1 liter ALO- Calculblcu.ft. CaCOSeed. Precip- Preciplated On Total 325 Mesh added charge itation itulcdEnd On Filling Volume in product Sample (gpl) (gpl) slurry (gpl) RatioLiquor of Slurry after attrition l 0.06 25 870 36.1 0.406 2.62 2.28 I122 I 0.00 870 37.0 0.404 2.64 2.31 l4.4

TABLE VII-continued COMPARATIVE PRECIPITATIONS WITH AND WITHOUT CaCOPrecipitation Conditions: (i) Wet seeding: Bayer Fine Seed 80% 325 meshadded as slurry in spent liquor (ii) 0.06 gpl CaCO added as indicated.(iii) Precipitated at I50F. for 24 Hrs. (iv) Filling Liquor: ll9.7/l86.30.643; T.T.S. 2.35.9 gpl. (v) Spent Liquor (with seed): 64.0/188.00.340; T.T.S. 242.5 gpl.

cc Filling liquor productivity in I liter M 0 Calculb/cu.ft. CaCO SeedPrecip- Preciplated On Total 325 Mesh added charge itation itated End OnFilling Volume in product Sample (gpl) (gpl) slurry (gpl) Ratio Liquorof Slurry after attrition TABLE VIII COMPARATIVE LABORATORYPRECIPITATIONS WITH AND WITHOUT CaCO Precipitation Conditions: (i) Wetseeding: Bayer Fine Seed of about 80% 325 mesh added as 30% slurry inspent liquor (ii) 0.06 gpl CaCO added, as indicated. (iii) Precipitatedat lF. for 24 Hrs. (iv) Filling liquor: l18.8/l82.l 0.652; T.T.S. 235.9gpl (v) spent liquor (with seed): 6l.2/l85.5 0.330; T.T.S. 249.l gplProductivity cc Filling I lb/cu.ft CaCO; Seed liquor AI O '71 325 Meshadded charge in l-Iiter Precipitated End Ratio on Filling on Total inproduct Sample (gpl) (gpl) of slurry (gpl) (calculated) Liquor Volumeafter attrition TABLE IX COMPARATIVE PRECIPITATIONS WITH AND WITHOUTCaCO Precipitation Conditions: (i) Filling liquor: 119.4/|s|.5 =0.658;T.T.S. 233 gpl. (ii) Dry seeding (Bayer Fine Seed 325 Mesh) (iii) 0.08gpl CaCO added, as indicated. (iv) Precipitated at I50F. for 24 Hrs.

7a -325 Mesh CaCO added Seed Charge A1 0; Precip- End Ratio Productivityin Product Sample (gpl) (gpl) itated (gpl) (calculated) lb/cu.ft. afterattrition The results reported in accompanying Tables VII and VIIIinvolve tests employing the precipitation of alumina trihydrate fromBayer pregnant liquor by wet seeding, i.e. 30% seed in Bayer spentliquor. The data reported in Table VII show that up to about 50 gpl seedcharge the liquor productivity increases with the seed charge. Abovethis, productivity based on filling liquor alone essentially levels offwhereas the productivities calculated on total volume of theprecipitation slurry starts to decline over 55 gpl seed charges. Thisdecline would appear to be due to the dilution of the spent liquorintroduced with the seed. Also, as indicated in the data of theaccompanying Table VII, for small seed charges between 25 and 55 gpl anaddition of 0.06 gpl CaCO resulted in products having 6-7 percentagepoints less 325 mesh particles. On the other hand, with large seedcharges between 55 and 85 gpl this difference was l0 percentage points,see Table VIII. With large seed charges of 55-85 gpl under dry seedingconditions, see Table IX, ever increasing liquor productivities wereobtained but the particle size difference of the alumina trihydrateproduct with CaCO: addition was only 5 percentage points less -325 meshas compared with alumina trihydrate precipitated in the absence of CaCOThis small, relative difference in particle size would appear to be dueto high starting ratios, i.e. no dilution of the filling liquor with theseed slurry.

Tests were also carried out to determine the superiority of calciumcarbonate addition over the addition of other calcium-containingcompounds, such as C210 and CaCl These tests shows that CaCO CaO andCaCl behave quite differently. In liquors of starting ratios above 0.610these compounds act as follows:

1. CaCO exerts the least inhibiting efiect on liquid productivity andgives consistently'best coarsening results, 7-10% on -325 mesh ascompared with blank samples.

2. CaO showed the greatest inhibiting effect on liquor productivitywithout coarsening of the alumina trihydrate products, and

3. CaCl tended to follow the action of CaCO but it also tended toinhibit precipitation more than CaCO giving erratic and inferiorcoarsening of the alumina trihydrate product.

Other tests illustrative of the advantages of the practices of thisinvention were carried out in Bayer plant liquors analyzing 190-200 gplcaustic (measured as Na CO 120-125 gpl alumina and 240-245 gpl totaltitratable soda (TTS).

The seed charge employed was 60 gpl fine seed alumina trihydratecontaining about 80% 325 mesh fraction. The precipitation was carriedout in one-liter Monel Bottles rotating end-overend at 14 RPM inconstant temperature water bath at 150F. for 24 hours.

These tests also showed that small additions of CaCO to seeded Bayeraluminate liquors have a coarsening effect on the precipitated aluminatrihydrate product, maximum coarsening being obtainable with a CaCOaddition of about 0.08 grams per liter, see the accompanying drawing.Improved results were otainable, however with CaCO additions as low as0.02 gram per liter and as high as 0.16 gram per liter. It was alsoobserved in these tests that the strengths of the precipitated aluminatrihydrate products, e.g. attrition resistance, increased withconcentration of added CaCO and that the relative effect of CaCO wasgreater when the precipitation slurries were subjected to vigorousmechanical mixing during the precipitation. Under the conditions ofvigorous mechanical agitation, all alumina trihydrate precipitationproducts were finer than comparative products obtained under mild mixingconditions, but the relative coarsening effect of CaCO addition was moreevident and more pronounced when 10 the comparative products wereobtained under identical vigorous mixing conditions.

As will be apparent to those skilled in the art in the light of theforegoing disclosure, many alterations, modifications and substitutionsare possible in the practice of this invention without departing fromthe spirit or scope thereof.

I claim:

1. A method of precipitating alumina trihydrate from filtered Bayerpregnant liquor saturated with respect to alumina trihydrate and havinga caustic content above about gpl caustic measured as Na CO to increasethe amount of alumina trihydrate having a particle size greater than 325mesh, which comprises adding to said filtered Bayer pregnant liquor fineseed alumina trihydrate as a precipitating agent for the precipitationof alumina trihydrate from said filtered Bayer pregnant liquor andsubstantially simultaneously with the addition of said fine seed aluminatrihydrate to said filtered Bayer pregnant liquor adding an amount offinely divided solid calcium carbonate in the range about 0.020.2 gpl ofsaid filtered Bayer pregnant liquor whereby the precipitated aluminatrihydrate has an increased amount of alumina trihydrate having aparticle size greater than 325 mesh as compared with the aluminatrihydrate precipitated in the absence of said added calcium carbonate.

2. A method in accordance with claim 1 wherein the fine seed aluminatrihydrate employed as a precipitating agent for the precipitation ofalumina trihydrate from Bayer pregnant liquor has a particle size suchthat at least about 60% by weight of said alumina trihydrate is smallerthan 325 mesh.

3. A method in accordance with claim 1 wherein said calcium carbonate isadded in an amount in the range from about 0.05 to about 0.1 gpl basedon said Bayer pregnant liquor.

4. A method in accordance with claim 1 wherein the caustic content ofsaid Bayer pregnant liquor measured as Na CO is in the range from aboutgpl to about 220 gpl.

5. A method in accordance with claim 1 wherein said fine seed aluminatrihydrate is added in an amount above about 50 gpl of said Bayerpregnant liquor.

6. A method in accordance with claim 1 wherein said fine seed aluminatrihydrate is added in an amount in the range from about 50 gpl to about90 gpl based on said Bayer pregnant liquor.

7. A method in accordance with claim 1 wherein said fine seed aluminatrihydrate is added in an amount above about 50 gpl of said Bayerpregnant liquor, said fine seed alumina trihydrate having a particlesize such that at least about 60% by weight of said alumina trihydrateis smaller than 325 mesh.

1. A METHOD OF PRECIPITATING ALUMINA TRIHYDRATE FROM FILTERED BAYERPREGNANT LIQUOR SATURATED WITH RESPECT TO ALUMINA TRIHYDRATE AND HAVINGA CAUSTIC CONTENT ABOVE ABOUT 160 GPL CAUSTIC MEASURED AS NA2CO3 TOINCREASE THE AMOUNT OF ALUMINATRIHYDRATE HAVING A PARTICLE SIZE GREATERTHAN 325 MESH, WHICH COMPRISES ADDING TO SAID FILTERED LAYER PREGNANTLIQUOR FINE SEED ALUMINA TRIHYDRATE AS A PRECIPITATING AGENT FOR THEPRECIPITATION OF ALUMINA TRIHYDRATE FROM SAID FILTERED BAYER PREGNANTLIQUOR AND SUBSTANTIALLY SIMULTANEOUSLY WITH THE ADDITION OF SAID FINESEED ALUMINA TRIHYDRATE TO SAID FILTERED BAYER PREGNANT LIQUOR ADDING ANAMOUNT OF FINELY DIVIDED SOLID CALCIUM CARBONATE IN THE RANGE ABOUT0.02-0.2 GPL OF SAID FILTERED BAYER PREGNANT LIQUOR WHEREBY THEPRECIPITATED ALUMINA TRIHYDRATE HAS AN INCREASED AMOUNT OF ALUMINATRIHYDRATE HAVING A PARTICLE SIZE GREATER THAN 325 MESH AS COMPARED WITHTHE ALUMINA TRIHYDRATE PRECIPITATED IN THE ABSENCE OF SAID ADDED CALCIUMCARBONATE.
 2. A method in accordance with claim 1 wherein the fine seedalumina trihydrate employed as a precipitating agent for theprecipitation of alumina trihydrate from Bayer pregnant liquor has aparticle size such that at least about 60% by weight of said aluminatrihydrate is smaller than 325 mesh.
 3. A method in accordance withclaim 1 wherein said calcium carbonate is added in an amount in therange from about 0.05 to about 0.1 gpl based on said Bayer pregnantliquor.
 4. A method in accordance with claim 1 wherein the causticcontent of said Bayer pregnant liquor measured as Na2CO3 is in the rangefrom about 180 gpl to about 220 gpl.
 5. A method in accordance withclaim 1 wherein said fine seed alumina trihydrate is added in an amountabove about 50 gpl of said Bayer pregnant liquor.
 6. A method inaccordance with claim 1 wherein said fine seed alumina trihydrate isadded in an amount in the range from about 50 gpl to about 90 gpl basedon said Bayer pregnant liquor.
 7. A method in accordance with claim 1wherein said fine seed alumina trihydrate is added in an amount aboveabout 50 gpl of said Bayer pregnant liquor, said fine seed aluminatrihydrate having a particle size such that at least about 60% by weightof said alumina trihydrate is smaller than 325 mesh.